1. Technical Field
The present invention relates to an image forming apparatus having an electrophotographic device, such as an electrostatic copier, a printer, or a facsimile, and an image forming method capable of suppressing so-called ghost (sometimes, it is called memory. Hereinafter, it is referred to as “external-additive memory”) caused by an external additive remaining after transfer.
2. Related Art
Generally, image forming apparatuses have been contrived which suppress ozone generation at the time of the charging of a photosensitive body and prevent deterioration of the photosensitive body caused by discharge products (for example, refer to Japanese Unexamined Patent Application Publication No. H04-275569). In the image forming apparatus disclosed in Japanese Unexamined Patent Application Publication No. H04-275569, the photosensitive body is largely negatively charged by a charging brush after transfer, and a positive corona charging is performed by corona charger, thereby adjusting the electric potential of the photosensitive body to a predetermined negatively charged potential.
Further, image forming apparatuses using toners with small particle diameters have been also contrived which are configured to clean a photosensitive body by removing external additive and residual toner after transfer (for example, refer to Japanese Unexamined Patent Application Publication No. 2004-101659). In the image forming apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2004-101659, the photosensitive body is largely negatively charged by a brush roller after transfer, thereby adjusting the charge polarities of the residual toner and the external additive which are not adjusted after transfer. In addition, the residual toner and the external additive are collected by a second brush roller to which a bias for attracting those has been applied.
In the image forming apparatus disclosed in Japanese Unexamined Patent Application Publication No. H04-275569, the transfer residual toner is removed in a cleaning section, but some of the external additive having a small particle diameter is not removed, and passes the cleaning section. The external additive is largely negatively charged when the photosensitive body has been negatively charged by using the charging brush. Hence, when the surface potential of the photosensitive body is adjusted by positive corona charging, the electric potential of the external additive is adjusted to be positively charged. However, some of the external additive, which is still negatively charged, remains amongst the external additive which have been adjusted to be positively charged. The reason is that, in the case of the negatively charged toner, an external additive having a high negative charging property is mixed with the toner in order to adjust the quantity of charge of the toner, and all the external additive having a high negative charging property are not positively charged even after undergoing the positive corona charging. In addition, the negatively charged external additive has an electrostatically repulsive interaction with the negatively charged toner, but since the quantity of charge of the external additive is significantly larger than that of toner base particles, the external additive electrostatically polarizes and attracts the toner base particles. Accordingly, the negatively charged external additive attracts the negatively charged toner in the developing area, thereby causing the external-additive memory.
In the image forming apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2004-101659, some external additive coming into contact with the brush bristles of the second brush roller becomes high-charged external additive with a high negative charge, but some external additive which does not come into contact with the brush bristles becomes low-charged external additive with a slight negative charge. The high-charged external additive is attached to the brush bristles, and is collected by the collecting roller which comes into contact with the brush bristles. However, the low-charged external additive is not attached to the brush bristles and has a small particle diameter, and thus passes the second brush roller. For this reason, the negatively charged external additive attracts the negatively charged toner in the developing area as described above, thereby causing the external-additive memory.